1. Field of the Invention
The present invention is related to the field of electromagnetic induction well logging instruments. More specifically, the present invention is related to methods of correcting the response of receivers in electromagnetic induction well logging instruments for the so-called "skin effect", and to verify the integrity of signals acquired by the receivers in an induction logging instrument.
2. Description of the Related Art
Electromagnetic induction well logging instruments are used to make measurements of the electrical resistivity of earth formations penetrated by wellbores. Induction well logging instruments typically include a sonde having a transmitter coil and one or more receiver coils at axially spaced apart locations from the transmitter coil. Induction well logging instruments typically include a source of alternating current (AC) which is conducted through the transmitter coil. The AC passing through the transmitter coil induces alternating eddy currents in the earth formations. In general, the magnitude of the eddy currents is proportional to the electrical conductivity (the inverse of the electrical resistivity) of the earth formations surrounding the instrument. The eddy currents in turn induce voltages in the receiver coils. The magnitude of the voltages induced in the receiver coils is generally proportional to the magnitude of the eddy currents. The conductivity of the earth formations therefore can be related to the magnitude of the voltages induced in the receivers.
The relationship of the magnitude of the eddy currents with respect to the conductivity of the earth formations is affected both by the frequency of the AC in the transmitter coil and by the conductivity of the earth formations themselves. The voltage actually induced in the receiver coils is typically less than what would be induced for any value of conductivity were the relationship between eddy current magnitude and the induced voltage a linear one. The difference between the voltage actually induced and the voltage which would have been induced were the relationship a linear one results from the so-called "skin effect".
Various methods of determining the magnitude of the voltages which would have been induced in the receiver coil absent the skin effect (thereby being able to determine the conductivity of the earth formations) are known in the art. A relationship, between the voltage induced in the receiver coils which is in-phase with the AC flowing through the transmitter coil with respect to the formation conductivity, is described in a paper by J. H. Moran and K. S. Kunz entitled "Basic Theory of Induction Logging and Application to Study of Two-Coil Sondes", Geophysics, vol. XXVII, no. 6, Part I (December), Society of Exploration Geophysicists (1962). The relationship described in the Moran and Kunz reference is typically limited to use in formation conductivities less than about 2 S/m at an AC frequency of about 20 KHz. At higher values of conductivity the magnitude of the in-phase voltage induced in the receiver is non-uniquely related to the conductivity, making it difficult to determine conductivity.
An improvement over the Moran and Kunz method is described, for example, in U.S. Pat. No. 3,147,429 issued to Moran. The Moran '429 patent describes a method of combining receiver signal components which are in-phase and 90 degrees out of phase (in quadrature) with the AC flowing through the transmitter, at a single frequency, in order to better estimate the skin effect magnitude. The method in the Moran '429 patent has proven to be difficult to use, particularly because it is difficult to measure accurately the magnitude of the receiver signal components in quadrature with the transmitter current. Further, the quadrature voltage magnitude is not only related to the conductivity, but is also affected by the magnetic and dielectric properties of the earth formations surrounding the tool, making determination of the conductivity by this method much more difficult.
U.S. Pat. No. 4,471,436 issued to Barber et al describes an implementation of the method described in the Moran '429 patent, but the implementation disclosed in the Barber et al '436 patent is still limited as to the effects of magnetic materials in the wellbore and the accuracy with which the quadrature signal can be measured.
An induction well logging instrument described in U.S. Pat. No. 5,175,605 issued to Chandler et al includes measurement of induction signals at two different frequencies. The apparatus in the Chandler et al '605 patent, however, does not provide for determining the frequency dependence of the skin effect, and so is limited as is the method of Moran '429 and Barber et al '436.
Another method of determining skin effect corrected conductivity is described in U.S. Pat. No. 4,611,173 issued to Bravanec et al, which includes adjustment of the frequency of the AC passing through the transmitter in response to the apparent conductivity of the earth formations. In higher conductivity formations, the magnitude of the skin effect can typically be reduced by operating the transmitter at a lower frequency. Lower operating frequency however; typically reduces the conductivity resolution of the instrument because for any particular value of formation conductivity, the magnitude of the voltage induced in the receiver absent skin effect is proportional to the AC frequency. It has also proven difficult in practice to adjust the operating frequency of the alternating current while the instrument is operating in a wellbore. This has made the system in the Bravanec et al '173 patent commercially impracticable.
A method is known in the art for estimating the frequency dependence of the skin effect by measuring skin effect at two frequencies and obtaining the difference between the skin effect at the two frequencies and extrapolating the difference to zero frequency. This method is described in A. A. Kaufman, "Resolving Capabilities of the Inductive Methods of Electroprospecting", Geophysics, vol. 43, no. 7, pp 1392-1398, Society of Exploration Geophysicists (1978). A drawback to the method in the Kaufman reference is that the differences in the skin effect at the two different frequencies is typically very small (except at high frequencies and high conductivities), which can result in large extrapolation errors. Another drawback to this method is that the apparent conductivity (receiver) signals can be noisy. Extrapolation to zero frequency of apparent conductivity values at only two frequencies is typically inadequate in the presence of significant amounts of noise to accurately represent the relationship of skin effect with respect to frequency.
Accordingly, it is an object of the present invention to provide a method of determining the skin effect magnitude which includes using response from a plurality of frequencies to better determine the frequency dependence of the skin effect, and thereby provide a more accurate determination of the true conductivity of the earth formation.
It is a further object of the present invention to provide a method for reducing noise and increasing accuracy in induction logging instrument signals by determining receiver response at a plurality of different frequencies and at a plurality of receivers so as to identify and adjust for noise originating in any of the receivers at any value of frequency.